The invention relates to joint packings, but more particularly, the invention relates to a packing with an elastomeric O-ring that is used to effect a high pressure seal between a cylindrical type bore and a piston-like member.
O-Ring type packings 10 provide an excellent and reliable high pressure and temperature seal between a smooth cylindrical bore 12 and a piston like member 13 fitted with an "O-ring" 14 as shown in FIG. 1. A preferred clearance between the piston member and cylindrical bore is always several times less than a chosen O-ring cross-sectional diameter 16. Publications are replete with recommended sizes of O-rings for various cylinder and piston diameters and applications. For example, a radial clearance of about 0.003 inch between a piston and bore is recommended for a 90 durometer O-ring to seal at 6000 psi. A recommended O-ring for a 4.5 inch diameter piston has a cross-sectional diameter of 0.210 inch. For such an application the O-ring cross-sectional diameter is 6 times greater than the recommended clearance. In high pressure applications, back up rings 18 may be used on either side of an O-ring to provide support. In any case, O-rings cannot be used as an effective sealing mechanism where there is radial clearance between an O-ring and a cylindrical bore because some O-ring precompression is required.